1. Field of the Invention
The present invention relates to an optical disk for recording, erasing or reproducing information by utilizing a laser beam, and more particularly to an optical disk arrangement for preventing substrate warpage when a plastic substrate is used therein.
2. Prior Arts
In a conventional optical disk of the above type, as shown in FIG. 10, a recording layer 21 is formed on one side of a plastic substrate 20 and is protected by a protective film 22. The recording film 21 has usually a four-layer or three-layer structure, and thus moisture hardly penetrates from the protective film 22 side to the plastic substrate 20. Accordingly, moisture penetrates to or desorbs from the substrate only in the other side of the substrate 20, i.e., the beam-incident side thereof. The absorption or desorption of moisture causes a local volume change of the substrate 20, leading to substrate warpage.
In the case of the optical disk having a plastic substrate, the substrate having a warpage to a significant extent is slanted relative to the optical axis of a converged light beam. In this case the converged light beam does not run along, the center of a guide groove even if the tracking servo mechanism is working, degrading signal quality. If the substrate is further warped, even the tracking servo mechanism becomes useless, and thus the disk itself becomes worthless. For this reason, it is required that the acceptable warpage degree be within a certain range. As shown in TABLE 1, there is a regulation of acceptable warpage degrees of optical disks.
TABLE 1 ______________________________________ Type of Optical Disk Warpage Degree ______________________________________ Compact Disk 10 mrad or less Write-Once type Disk 5 mrad or less Magneto-Optic Disk 5 mrad or less ______________________________________
As for compact disks, warpage is acceptable to a relatively high degree as compared with the case of other types of disk because the number of revolutions required for the disks is relatively small (200-500 rpm) and thus the tracking servo mechanism or focusing servo mechanism can sufficiently align a light-beam with the groove.
As for write-once type disks and rewritable disks (magneto-optical disks), warpage is limited to a relatively low degree allowing for the beam-alignment capability of the servo mechanism or focusing servo mechanism because such disks are required to turn at high speed (for example, 1800-3600 rpm) in order to realize a higher data-transfer rate. However, it has been difficult to minimize the warpage degree when a single substrate is used. Therefore, it has been a conventional practice to bond a single-substrate disk to another to form a both-face type disk, thereby reducing the warpage degree.
Recently, an attention has been attracted to an overwrite technique particularly for magnetooptical disks, and thus the needs for single-substrate disks have been increased. For rewriting data, a conventional magnetooptical disk requires to be turned twice, once for erasing the existing data; once for recording new data. In contrast a magnetooptical disk using the overwrite technique requires to be turned only once for both erasing the existing data and recording new data, thereby improving the data transfer rate.
Among various modes being studied for the overwrite technique, a magnetic-field modulation is considered to be promising. A conventional overwrite technique uses light modulation and performs information recording on the basis of ON-OFF of light with the magnetic field direction kept constant. On the other hand, an overwrite technique using the magnetic-field modulation performs information recording by changing the magnetic field direction.
In the case of overwrite technique using magnetic-field modulation, the magnetic field direction must be changed at high speed, and high-speed magnetic field modulation with a minimized electric consumption of an electromagnet requires to minimize the distance between the electromagnet and the recording film. With the abovementioned both-face type disk, it is difficult to minimize the distance between the recording film and the electromagnet because one substrate intervenes therebetween. Accordingly, a single-substrate type optical disk becomes necessary, as stated above.
In the case of using a plastic substrate in a single-substrate type disk, warpage of the substrate becomes a problem, as noted above. Warpage caused immediately the fabrication of a disk has been reduced to sufficiently meet the regulation of Table 1. However, it has been recently found that optical disks are prone to further warp during their operation. That is, the transaction of Japanese Applied Physics Meeting, p. 872 (1988/spring) reported that transitive warpage occurred during the change of environmental conditions. For instance, when the humidity varies from 60.degree. C., 90% RH to 60.degree. C., 50% RH, the warpage degree reaches beyond 10 mrad at maximum.